import java.util.*;

class Solution {
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {

        ListNode ret = new ListNode(0);
        ListNode cur = ret;
        while (list1 != null && list2 != null) {
            if (list1.val <= list2.val) {
                cur.next = list1;
                cur = cur.next;
                list1 = list1.next;
            } else {
                cur.next = list2;
                cur = cur.next;
                list2.next = list1;
            }
        }
        if (list1 == null) {
            while (list2 != null) {
                cur.next = list2;
                cur = cur.next;
                list2 = list2.next;
            }
        }else{
            while (list1 != null) {
                cur.next = list1;
                cur = cur.next;
                list1 = list1.next;
            }
        }
        return ret.next;
    }
}